This invention relates generally to the disinfection of dental water unit lines, using aqueous chlorine dioxide solutions, and more particularly to the reduction and destruction of bacteria in biofilms which form on the inner walls of such water lines.
The dental profession is becoming increasingly aware, and concerned, that small diameter pipes carrying fresh water from operatory equipment to their patients are contaminated by bacteria and other microorganisms contained in the water flowing through them. Some of the microorganisms inevitably adhere to the inner walls of the pipes and accumulate together with microscopic sediments or other substances into what is commonly known as a biofilm. The biofilm quickly and tenaciously coats the inner walls of the pipes. As it grows, it absorbs nutrients from the water, becoming a culture medium for more microorganisms. These films are typically 30-50 microns thick, and the microbes are distributed throughout the biofilm matrix. Many different types of micro-organism are found in these films including bacteria, fungi, algae and amoebae.
The bacteria found in biofilm are primarily of environmental origin such as Bacillus, Pseudomonas and related genera, and Corynebacterium, but can include organisms from dental patients which are capable of causing serious human infections such as Legionella. There have also been reports of the isolation of human oral bacteria, presumably from back-flow through dental instruments attached to the water line. Sloughing off of microbial aggregates from these biofilms into the lumen will result in bacterial populations reaching alarming levels in the water discharge from the dental instruments connected to the fresh water line. The average bacteria count in the water discharge of dental instruments is approximately 200,000 [2xc3x97105] colony forming units per milliliter (cfu/ml) and in some extreme cases can reach 10,000,000 [107] cfu/ml. This is of particular concern since dentists, dental surgeons and dental hygienists, as well as many of their patients, are well aware of the importance of meticulously sterilizing dental instruments to minimize bacterial contamination of these patients. In particular, since dental instruments are used directly in a patient""s mouth, when bleeding may sometimes occur as a result of a dental procedure, it is of paramount importance to minimize the presence of microorganisms carried by dental instruments. The microorganisms can of course range from relatively harmless bacteria to dangerous pathogens. Thus, efforts are continuously made to remove microorganisms from dental instruments and from the fresh water lines feeding dental instruments. These include such equipment as air/water guns, high speed water turbines and ultrasonic tartar removers. Where applicable, thermal sterilization remains one of the best methods for eradicating the presence of microorganisms, such as for most hand held dental instruments. However, thermal sterilization is obviously not practical for the decontamination of fresh water lines, which continue to be inordinately difficult to clean, and maintain free from microorganisms.
Many efforts have been expended to accomplish this reduction/destruction, although none has proved completely satisfactory. For example, in the most obvious approach, it has been suggested to use sterile water, particularly to drain the fresh water lines during periods of non-use. Or, as taught in U.S. Pat. Nos. 5,360,338, 5,824,323, and 5,942,125 to create sterile water in the equipment itself, such as by ozonization. Besides the expense, the considerable effort to accomplish these complex operations makes such procedures realistically unacceptable. It is also known that a detergent such as polyoxyethylene sorbitan monooleate (Tween 80(trademark)) at approximately 4% dislodges biofilm from small diameter water lines used in dental equipment. However, the use of detergent alone does not effectively destroy the microorganism population. Even the teachings of U.S. Pat. No. 5,942,480, combining detergent, denaturing agent and antimicrobials have not been adopted by dental practitioners as being an effective solution to the elimination of bacterial biofilm.
A number of patents, such as U.S. Pat. Nos. 5,971,757, 5,961,326, 5,749,726, and 5,204,004 teach the use of a variety of replaceable in-line water filters to trap bacteria, such as from biofilm sloughing. These approaches have been found to be impractical, generally because of rapid clogging of the filters whose pore-sizes are sufficiently small to trap bacteria, and which also trap shed biofilm fragments and scale. Since the biofilms in the water lines continue to build up, even in the presence of disinfecting agents, none of those methods has been shown to effectively remedy the microorganism proliferation for any length of time.
It is also known in the art that disinfectants, such as povidone-iodine at a concentration of approximately 10%, reduce the number of microorganisms in small diameter water lines. It is further known from U.S. Pat. No. 5,942,480 that a mixture of mandelic and lactic acids reduces the number of susceptible microorganisms in contaminated tubing. However, such disinfection is somewhat superficial since it fails to effectively attack and destroy the microorganisms found in the biofilm. Consequently, the disinfection effect is short-lived. After 24 hours of treatment with povidone-iodine, the numbers of bacteria are greatly reduced but quickly begin to rise after eight days. Related systems, such as those using iodinated ion-exchange resins, as in U.S. Pat. Nos. 5,556,279 and 5,320,624, also suffer from the same problem, i.e. the inability to penetrate the biofilm and destroy the resident organisms.
In an effort to penetrate and remove biofilms and related deposits, the Ultra-Kleen(trademark) company markets a product based on an alkaline peroxide and a phase transfer catalyst, as taught in U.S. Pat. Nos. 4,847,089 4,941,989, 5,320,805 and 5,620,527. The product makes a claim as a specialty cleaner for dental unit water lines, rather than a more stringent disinfectant or sterilant. Several more potent germicides have been evaluated, including bleach and acidified bleach (U.S. Pat. No. 6,019,905), glutaraldehyde and an alkaline glutaraldehyde-phenolic disinfectant (U.S. Pat. No. 6,040,283), cetyl pyridinium chloride, peracetic acid, chlorhexidine and isopropanol. A recent report in the Journal of the American Dental Association (January, 1999) showed that 15-hours contact with household bleach (5.25%), glutaraldehyde (3%), or isopropanol (15.3%) resulted in effluents from biofilms that were free of recoverable bacteria, but that recolonization returned to pretreatment levels by day 3 for glutaraldehyde, day 6 for bleach, and day 15 for isopropanol. The report stated that the residual effect of these agents raises concerns about the slow release of potentially toxic substances from the residual biofilm matrix into water reaching the dental patient. Furthermore, while the agents temporarily reduce microorganisms in effluent water, they do little to destroy the biofilm matrix in the water lines, even with periodic treatments. A subsequent article suggested that weekly treatment with 5000 ppm chlorine (diluted bleach) coupled with the use of 3 ppm chlorinated water in the dental unit consistently attained the desired bacterial level of  less than 200 cfu/ml. Two concerns arise however with this approach. One is that chlorine has been shown to elevate levels of trihalomethanes (THMs) and other chlorinated byproducts in water, where the EPA has placed limits on THMs in water. Additionally chlorine is a strong oxidant, and causes corrosion of most metals used in operatory equipment. While a 3 ppm level is of little concern, chlorine at the 5000 ppm level has significant corrosion potential for components of expensive dental equipment.
A so-called Stabilized Chlorine Dioxide (SCD) mouthrinse has recently been evaluated for the decontamination of the water lines of ultrasonic scaling units in dental operatories (Wirthlin et al., J Periodontol., 2001, 72:401-410). Although the article title refers to the use of a 0.1% chlorine dioxide/0.5% sodium phosphate mouthrinse, a later reference in the paper correctly describes the product as a stabilized chlorine dioxide formulation. This has been confirmed by both the label on the product, ClõSYSII, which lists stabilized chlorine dioxide in the ingredients, and by analysis of the product, which revealed the lack of any detectable chlorine dioxide (i.e. it had below 0.5 parts per million [ppm]), despite the publication""s reference to 1000 ppm (0.1%) of chlorine dioxide in the rinse. As is usual for SCD products, which are in fact stabilized chlorite solutions, the sodium chlorite concentration of the product was found to be predominant, at 0.173%, corresponding to 0.129% of chlorite ion. The assumption with SCD products is that conversion to ClO2 from chlorite (i.e. ClO2xe2x88x92) is 80% efficient, via the chlorous acid route, so the result of 80%xc3x970.129% chlorite, or 0.102% xe2x80x9cchlorine dioxidexe2x80x9d is consistent with the nominal 0. 1% chlorine dioxide in the product. In the results of the paper on ultrasonic scaling unit waterline decontamination, the decrease of biofilm bacteria following 30 minutes of contact with the SCD was insufficient to offer much promise, amounting to less than 1-log cfu/ml of bacterial reduction, as compared with water flushing along. Specifically there was a 3-5 fold reduction with tap water flushing, and 12-20 fold reduction with the SCD. For dental water lines typically delivering 3-5 logs/cfu of organisms in their water, from resident biofilms, the reduction offered by the chlorite mouthrinse is markedly insufficient for alleviating this pervasive problem.
Accordingly there remains a long-felt need for compositions and their methods for use to decontaminate small diameter water lines for dental equipment and effectively dislodge and eliminate adherent biofilms. At the same time such desired compositions and methods will destroy the microorganism flora in the fresh water and in the dislodged biofilm without concomitant adverse effects on dental equipment, or formation of potentially harmful substances in the water which reaches the patient. There is a more particular need to identify compositions and their methods of use which are compatible with current dental operatory practice, so as to minimally interfere with normal procedures. And, at the same time they should provide the reassurance to the dental professional that the water effluent from such dental units poses no risk from unwanted pathogens or chemical residues or transformation products derived from the decontaminating antimicrobials.
It is an object of the present invention to provide compositions and methods which can be used to decontaminate small diameter water lines for dental equipment and effectively dislodge and eliminate adherent biofilms.
It is also an object of the present invention to provide such desired compositions and methods which will destroy the microorganism flora in the fresh water and in the dislodged biofilm without concomitant adverse effects on dental equipment, or formation of potentially harmful substances in the water which reaches the patient.
It is also an object of the present invention to provide means for safely destroying microbial-laden biofilms, and ensuring water supplies to the patient that are consistently pathogen and contaminant free.
Any one of these and/or other objects of the invention may be readily gleaned from a reading of the description of the invention which follows.
In brief, the present invention is directed to disinfecting compositions for dental unit water lines, particularly effective against biofilms which form on the luminal walls of the piping and reservoir components of dental equipment, as well as methods related to the use of such compositions to reduce microbial numbers in water-bearing dental equipment and maintain reduced levels on a continuous basis.
Compositions according to the present invention in a decontamination embodiment comprise an effective amount of chlorine dioxide within the range of about 100 ppm (parts per million) to about 2,500 ppm, preferably within the range of about 250 ppm to about 1,500 ppm, more preferably about 500 ppm to about 1,000 ppm and about 1 ppm to about 10 ppm when used in a maintenance regimen. Methods according to the present invention comprise exposing contaminated dental unit water lines to an effective amount of a chlorine dioxide solution comprising chlorine dioxide within the range of about 100 ppm to about 2,500 ppm and for a period of time effective to reduce microbial growth in biofilm lining the dental unit water lines.
The present invention also relates to methods for treating biofilms in dental water lines comprising exposing biofilms lining dental unit water lines to an effective amount of chlorine dioxide in a manner and for a time effective to reduce microbial growth and microbial population and ultimately, reduce or eliminate the biofilm.
In one embodiment, dental units are decontaminated with an initial exposure (about 10 to about 16 hours) to an effective amount of an aqueous chlorine dioxide or chlorine solution, and thereafter to weekly exposures of an effective amount of a chlorine dioxide containing composition (preferably, about 1000 ppm) for a time and in a manner (preferably, about 30 minutes weekly) which is effective to reduce microbial growth and the size of a biofilm in which the microbes grow. In many instances, the biofilm bacteria may be eliminated after an initial exposure to the chlorine dioxide containing composition. In another embodiment, the units are decontaminated in the same manner by an initial overnight exposure to 1000 ppm, followed by exposure for three consecutive days to a 100-500 (preferably 100) ppm solution of chlorine dioxide on a weekly basis. In other embodiments, the dental units may be decontaminated daily with effective amounts of chlorine dioxide and/or chlorine solutions. In yet another embodiment, after an initial decontamination, a dental unit is fitted to deliver a continuous supply of less than about 1 ppm to about 10 ppm (preferably, up to about 1 ppm) chlorine dioxide solution to the water directed to the patient. And, in a related embodiment, after an initial decontamination or in instances where the dental unit is not yet contaminated, for example, in instances where the unit is new or has recently been repaired or serviced, the dental unit can be put on a maintenance treatment of up to 3-10 ppm of chlorine dioxide, included in the supply water from an external source.
These, and other aspects of this invention will become evident upon reference to the following detailed description.
The following terms shall be used to describe the present invention.
The term xe2x80x9cchlorine dioxidexe2x80x9d is used to describe the chemical compound ClO2 Chlorine dioxide is a potent antimicrobial agent, as well as a bleaching material, in many commercial and industrial applications. It has disinfectant, bleaching and oxidizing properties. ClO2 is a reactive gas which is explosive in air at levels approximating 10%. It is generally produced xe2x80x9con sitexe2x80x9d by acidification of chlorite solutions, but may be stabilized in certain compositions. Chlorine dioxide compositions, when used in decontamination aspects according to the present invention generally comprise about 25 ppm (preferably at least about 100 ppm) to about 2,500 ppm, more preferably about 500 to about 1,500 ppm, even more preferably about 1,000 ppm of chlorine dioxide and less than about 1 ppm (preferably, at least about 1 ppm) to about 10 ppm in maintenance aspects according to the invention.
The term xe2x80x9csmall diameter water linexe2x80x9d is used throughout the specification to describe water line tubing (which may be made of any material, but is usually made of stainless steel, a copper alloy or a high density plastic) in a delivery system, including dental water lines, which has a small diameter (preferably, less than about 0.75xe2x80x3, more often less than about 0.5xe2x80x3 or even less than 0.25xe2x80x3), which is generally used in intermittently static water-containing and delivery systems and over time, becomes contaminated with microbial growth which ultimately results in a biofilm on the inside walls of the water lines. In one particularly preferred aspect according to the present invention, the term xe2x80x9csmall diameter water linexe2x80x9d also applies to the piping and reservoir tanks of dental units.
The term xe2x80x9cbiofilmxe2x80x9d is used to describe a microbial film which appears on the inner walls of dental water lines and which is treated (most preferably, removed) using compositions according to the present invention. Biofilms according to the present invention may range in depth from a thin film to rather substantial films (biofilms up to 1 mil thick are not uncommon) and may comprise huge numbers of microorganisms which exist in a film coating the inner walls of small diameter water lines, and such biofilms may vary in length from a short span or small area to, in certain cases, the complete span of a water line.
The term xe2x80x9ceffectivexe2x80x9d is used to describe an amount or concentration of an agent such as the composition which contains chlorine dioxide in the present invention or the amount of time used to produce an intended result, the ultimate result being the reduction in microbial flora which are found in biofilms. Consistent with the reduction and/or elimination of microbes, the size of the biofilm is reduced (preferably, and in most instances, the biofilm is completely eliminated from the small diameter water line). For example, in the case of the reduction and/or the complete removal of a biofilm from a water line surface, an effective amount of the composition according to the present invention is that amount which is effective for substantially eliminating microbial growth and in this manner, reducing the thickness and/or the size of the biofilm to a level where it is completely removed. In general, an effective time (also, a sufficient time or sufficient duration) is the amount of time in which a composition according to the present invention is exposed to a biofilm in order to effect reduction in the actual number of microbes within a biofilm. Thus, compositions according to the present invention are used to treat biofilms in an effective amount and for an effective period to reduce and/or eliminate microbial flora in a biofilm, the ultimate result being the reduction in the size and preferably, the elimination of a biofilm from a small diameter water line. In general, chlorine dioxide compositions will range from about 25 to about 2,500 ppm chlorine dioxide in decontamination steps and from less than about 1 ppm (preferably, at least about 1 ppm) to about 10 ppm chlorine dioxide for maintenance steps. In the case of the use of chlorine solutions, which may be used in decontamination steps or maintenance steps in the present invention (along with at least one decontamination or maintenance step which utilizes chlorine dioxide), in decontamination applications effective amounts of chlorine range from about 1000 ppm to 130,000 ppm or more, preferably at least about 5,000 ppm within this range and in maintenance applications effective amounts of chlorine range from about 1 ppm to about 10 ppm, preferably about 1 ppm to about 3 ppm. The terms xe2x80x9ccontinuousxe2x80x9d and xe2x80x9ccontinualxe2x80x9d within the context of their use, shall mean on a regular, generally daily basis with intermittent cessation for periods for maintenance or non-use.
The term xe2x80x9creducexe2x80x9d or xe2x80x9creductionxe2x80x9d shall be used to describe the degree that chlorine dioxide compositions according to the present invention diminish the number of microorganisms which are found in biofilm. The method according to the present invention may be used to reduce the number of microorganisms in biofilm by at least 50%, more preferably at least 75%, even more preferably at least about 90-95%. In certain applications, microbial numbers will be reduced by at least 98+%. It is an unexpected result that compositions according to the present invention will reduce microbial flora in biofilms to such an extent, even with only one treatment. The term xe2x80x9celiminatexe2x80x9d or xe2x80x9celiminatingxe2x80x9d shall mean reducing microbial growth in biofilm to a level of less than about 1% of the original microbial population.
The present invention is generally directed to disinfecting compositions comprising chlorine dioxide which are effective in penetrating bacterial biofilms that develop and populate the inner wall tubing of intermittently static water-containing and delivery systems, and markedly reducing or destroying all microbial populations therein. In preferred aspects, the present invention is particularly directed to the piping and reservoir tanks of dental water units. Biofilms, which form rapidly through a combination of active and passive retractions and colonization by bacteria from the public water systems, can harbor potential pathogens, including protozoa, and may pose particular risk to immunocompromised dental patients. The present invention represents an unexpected result in providing an effective means of reducing and/or eliminating biofilms from small diameter water lines, inasmuch as other common disinfectant methods do not readily reduce and/or eliminate biofilms.
The carbohydrate matrix of the biofilms is of such structure that larger germicidal molecules, such as glutaraldehyde and chlorhexidine cannot effectively penetrate the sponge-like structure of the film to reach and destroy the resident microbes. Chlorine, on the other hand, is a very effective antimicrobial that appears to exert its antimicrobial effect by oxidatively-destroying accessible portions of the carbohydrate-based biofilm, while simultaneously killing the microbes in those sections. Being a strong oxidant, it cannot effectively penetrate the film and selectively destroy the organisms, but oxidizes all exposed organic matter, biofilm and organism. In one facility, 13% (130,000 ppm) hypochlorite has been used periodically to disinfect the dental units, which provides dramatic reductions in bacterial counts, but these rise again rather quickly presumably because the biofilms are not eliminated completely. This is consistent with the finding that biofilm bacteria are 150-3000 times more resistant to hypochlorous acid (free chlorine) than are unattached cells, and that transport of chlorine into the biofilm is the major rate limiting factor in disinfection. Increasing the level of chlorine, even to 130,000 ppm, does not increase disinfection efficiency.
In contrast and unexpectedly, chlorine dioxide (ClO2), which is a comparably small molecule, has the apparent ability to penetrate biofilms with a much greater efficiency and destroy deep-lying bacteria. The probable basis for this penetrability is the lower oxidation potential of ClO2 as cf. the two forms of aqueous chlorine, i.e.
HOCl+H++2exe2x88x92xe2x86x92Clxe2x88x92+H2O 1.49V
Cl2+2exe2x88x92xe2x86x922Clxe2x88x921.36V
ClO2+exe2x88x92xe2x86x92ClO2xe2x88x920.95V
The lower potential for oxidation allows the ClO2 molecule to diffuse through much of the glycocalyx of the biofilm with little interaction and reductive degradation by the carbohydrate film, and thus reach the protein-bearing surfaces of the bacteria and effect their oxidative destruction. This lower tendency for ClO2 reaction with the glycocalyx, coupled with its high antimicrobial efficacy, is the prime reason why ClO2 may be more effective here.
An expert panel of the American Dental Association has recommended that the dental industry in the US develop methods capable of achieving fewer than 200 (2xc3x97102) cfu/ml in unfiltered water from dental units by the year 2000. Current European Union legislation requires that water emanating from dental water units be of potable quality. The EU requires that total bacterial counts in drinking water be 100 cfu/ml or lower. This is to be compared with current bacterial effluent levels ranging from 105 to 107, primarily stemming from biofilm sources, representing a thousand-fold to hundred thousand-fold higher level than recommended. Typical potable water sources have bacterial levels in the xcx9c103 to 105 cfu/ml range, as a broad generalization, although some municipal water supplies do achieve the 102 cfu/ml level recommended by the various commissions. Practically speaking then, a 100-fold reduction in bacterial levels (i.e. 102 cfu/ml) which reach the dental patient is a practical goal. This represents, in essence, reducing the effluent bacterial levels to that of the incoming water.
The first step in achieving that goal is to destroy the source of the organisms, i.e. the biofilm, responsible for the elevated counts above that of the incoming water. This requires contact with an effective germicide over a sufficient time span to eliminate the bacterial population of the film, if not the film itself. Once that has been accomplished, the water supply should be so treated that redevelopment and/or repopulation of the biofilm is suppressed.
Since biofilms accumulate in virtually all dental water units, but to different degrees depending on the manufacturer and model and the number of individuals using the units in a dentist""s office, the following general approach is recommended for both Phase 1, the initial decontamination phase, and Phase 2, the maintenance phase (maintaining bacterial levels comparable to that of the potable supply water). For the initial destruction of organisms resident in the biofilm adhering to the inner surfaces of contaminated dental water units, an aqueous solution of chlorine dioxide (ClO2) in the concentration range of 100 to 1000 parts per million (ppm) is preferred. The contact time and number of decontamination cycles will vary, depending upon the degree of contamination, unit model and manufacturer, and other relevant factors such as microbial counts of the water source and the nature of the bacterial populace. Resident times for the solution will vary from approximately 30 minutes to 24 hours or longer, with a preferred exposure being overnight (e.g. 10 and 16 hours). The ClO2 solution can be generated by any of the standard methods, such as oxidation of a metal chlorite solution by hypochlorite, persulfate, or the like, or reduction of a metal chlorate solution. (See, for example, Massschelein, W. J.; (1979) Chlorine Dioxde; Chemistry and Environmental Impact of Oxychlorine Compounds. Ann Arbor Science, Michigan). In a preferred embodiment, the ClO2 is prepared by oxidation of a chlorite solution of such concentration that the molar ratio of residual chlorite to formed ClO2 is xcx9c1:1 to xcx9c20:1 [for example, as disclosed in U.S. Pat. No. 5,993,864, relevant portions of the disclosure of which are incorporated by reference herein]. The chlorine dioxide containing solution may be prepared on site, or delivered from a storage container containing pre-prepared solution. Frequency of application of the phase 1 treatment will generally range from a single treatment, in Dental Water Units (DWU) of lesser biofilm contamination, to a five-cycle treatment for severely contaminated lines. In the latter cases, overnight residence of the ClO2 solution is recommended for each application.
For the phase 2 maintenance phase, a level of ClO2 of up to about 1 ppm is recommended in the water that reaches the patient. In a preferred embodiment, the ClO2 is delivered as a dilution of a concentrate delivered from an external unit with a proportioning device. The ClO2 in that unit may be either prepared directly prior to connecting it into the water line, from an appropriate chemical system, or purchased pre-prepared in certain ClO2-impermeable containers. The degree of ClO2 concentration may range up to several thousand ppm, depending upon such factors as its mode of manufacture, proportioning ratios, stability of the container to ClO2 solutions, and availability of prepackaged material.
The treatment solution may be placed, for example, in the reservoir of the equipment or, in some recent models, into an injection port for that purpose. In a preferred embodiment, the preformed ClO2 solution in a special ClO2-impermeable container comprising glass or a polyethylene terephthalate container (such as those described in U.S. patent application Ser. No. 09/300,505, filed Apr. 28, 1999, entitled xe2x80x9cCompositions and Methods for Storing Aqueous Chlorine Dioxide Solutionsxe2x80x9d, relevant portions of which are incorporated by reference herein) adapted to be attached through a special fixture directly to the dental water line, is connected to the line, and allowed to flow into the water through an appropriate toggle switch.
Alternatively, the ClO2 can be generated directly in the reservoir by pre-insertion of the reactive oxychlorine species, e.g. the chlorite, followed by addition of the components necessary to bring about conversion to ClO2, e.g. measured amounts of acid buffering agent and oxidant. This applies for both the phase 1 (decontamination) or phase 2 (maintenance) operations. It is important that the treatment water reach all surfaces of the piping of the equipment which may contain biofilm, including, where appropriate the high-speed drill and the two water-bearing lumens in the so-called three-in-one, i.e. the sucker and the water spray. For phase 1 applications, the ClO2 solution can contain an oxidation-resistant coloring material (e.g. methylene blue) to visually verify that the solution has fully run through the various pipings prior to sealing the system for the desired contact period. In both treatment phases, following application and residence (in phase 1), the ClO2 is flushed from the unit with multiple volumes of water relative to the dead volume of the unit.
To verify the efficacy of both treatments, the effluent and supply waters are then sampled periodically by standard microbiological procedures known to those skilled in the art. In a typical sampling, the waters would be serially diluted and inoculated into the following media:
If the effluent water microbial counts are not comparable to that of the supply water, the ClO2 concentration and/or contact time of the treatment is appropriately increased, and the flushing and evaluation is then repeated.
The concentration of ClO2 to be used in the decontamination phase can range from about 100 ppm to 2,500 ppm, depending on many factors, which include the degree of biofilm build-up, age of the system, make and model of the equipment, relative degree of down-time of the equipment, the microbial population of the biofilm and other characteristics of the supply water (e.g. temperature for preceding period, hardness, organic matter). Residence time of the disinfectant is also dependent on many of the same characteristics, but it is generally preferred to select an overnight exposure, using a ClO2 solution containing about 500 to about 1,500, more preferably about 1,000 ppm of ClO2. If subsequent culturing (measurement) shows organism counts above that of the influent water, the treatment is repeated, at a concentration dependent on the count disparity, and resampling and measurement then repeated. The concentration of ClO2 to be used in the maintenance phase can range up to 1 ppm, consistent with any pertinent federal regulations (e.g. the US Environmental Protection Agency). An alternative maintenance solution for phase 2 is chlorine, in a concentration up to about 3 ppm, which is the level at which chlorine is minimally detectable by taste or smell). This can be achieved, for example, by chlorinating municipal waters which are not already chlorinated or by supplementing those that are, since municipal water chlorination levels generally are lower than 3 ppm. Means of accomplishing such chlorination are well known to those of ordinary skill and fully known to those skilled in the art.
The present invention is illustrated by the following examples, which are to be regarded as illustrative rather than restrictive. Unless otherwise noted, all parts and percentages in the examples, as well as the specification and claims, are by weight.